1. Field of the Invention
The present invention relates to a driving device for an ultrasonic wave motor in which a movable member is driven by travelling vibration wave generated in a resilient member by a piezoelectric member.
2. Related Background Art
As disclosed for example in the Japanese Patent Application Laid-open No. 59-111609, the ultrasonic wave motor utilizing travelling vibration wave causes a bending vibration in a piezoelectric member by applying an AC voltage thereto, thus generating a travelling vibration wave in a resilient member on which said piezoelectric member is attached, and drives by friction a rotor maintained in contact with said resilient member.
The speed reduction in such ultrasonic wave motor has been achieved either by switching the frequency of the applied AC voltage to a frequency corresponding to the desired lower speed, or by switching said AC voltage to a lower voltage.
However said frequency switching method is unable to cancel the inertia of the load connected to the rotor at the switching of frequency of the AC voltage and requires a long decelerating time, because:
(1) there is a delay in time, after the frequency switching, before the amplitude of the vibration wave generated in the resilient member reaches a value corresponding to the lower speed; and
(2) in the presence of the vibration wave, the resilient member and the rotor have low frictional force therebetween as they mutually contact at the peaks of the vibration wave.
Also said AC voltage switching method has a drawback of requiring a long decelerating time due to similar reasons.